That came out this year... Doki Doki Literature Club, which is free (still free if you'd rather get it via Steam). It's fairly short, it's clever, and you should go into it knowing as little as possible...which is why it might seem like people undersell it. Suffice to say that it's actually a psychological horror game, hence the warning on the sites there or displayed before the game starts.

I also played/enjoyed VA-11 Hall-A this year, though it came out last year.

Cuphead. But I don't love it like most. I went through a bunch of Contra this year and there's just no comparison for me, but I appreciate Cuphead's aesthetic a lot.

My favorite indie game overall this year that didn't release in 2017 would be Momodora: Reverie Under the Moonlight. Felt a lot like an SNES game with a little bit of Dark Souls atmosphere influences in there, loved it.

I found Last Day of June to be a fantastic little game. I could see this one appealing heavily to fans of To The Moon and it's sequel Finding Paradise (another one that came out late last year which I've not played it yet, but am definitely looking forward to). The entire game is based on the Steven Wilson song Drive Home, and most particularly the accompanying video for that song.

I wouldn't call myself a Steven Wilson fanboy per se, but I'm rather fond of much of his work, and it's definitely the initial factor that drew me to this game. If you can check at least two of the following boxes, I think you might dig this one.